Лекция №1.8. Exceptions.

1. Abnormal program termination.
2. Return error codes.
3. Macro errno.
4. try/catch/throw.
5. Using objects as exceptions.
6. Exception specifications.
7. Unwinding stack.
8. More exception features.
9. RAII.
10. Exceptions in constructors.
11. Best practices.

Transcript

1. ОБЪЕКТНО- ОРИЕНТИРОВАННОЕ ПРОГРАММИРОВАНИЕ Лекция № 1 / 08
2. EXCEPTIONS
3. ABNORMAL PROGRAM TERMINATION ABORT() ... double hmean(double a, double b) { if (std::abs(a+b) < epsilon) { std::cout << "untenable arguments to hmean()\n"; std::abort(); } return 2.0 * a * b / (a + b); } ...
4. ABNORMAL PROGRAM TERMINATION ABORT() ... int main() { double x, y, z; std::cout << "Enter two numbers: "; while (std::cin >> x >> y) { z = hmean(x, y); std::cout << "Harmonic mean of " << x << " and " << y << " is " << z << '\n'; std::cout << "Enter next set of numbers <q to quit>: "; } std::cout << "Bye!\n"; return 0; }
5. RETURN ERROR CODES ... bool hmean(double a, double b, double* ans) { if (std::abs(a+b) < epsilon) { *ans = std::numeric_limits<double>::infinity(); return false; } *ans = 2.0 * a * b / (a + b); return true; } ...
6. MACRO ERRNO ... int main() { double not_a_number = std::log(-1.0); if (errno == EDOM) { std::cout << "log(-1) failed: " << std::strerror(errno) << '\n'; } return 0; }
7. • Сигнализирование об ошибках в C: • Код возврата функции. • Глобальная переменная (errno). • Можно проигнорировать! • Исключения нельзя проигнорировать! Muahahaha EXCEPTIONS
8. • Генерация исключения throw. • Перехват исключения обработчиком catch. • Использование блока try. EXCEPTIONS
9. ... double hmean(double a, double b) { if (std::abs(a+b) < epsilon) { throw "bad hmean() arguments: a = -b not allowed"; } return 2.0 * a * b / (a + b); } ... EXCEPTIONS
10. ... int main() { double x, y, z; std::cout << "Enter two numbers: "; while (std::cin >> x >> y) { try { // start of try block z = hmean(x,y); } // end of try block catch (const char* s) // start of exception handler { std::cout << s << '\n'; std::cout << "Enter a new pair of numbers: "; continue; } // end of handler std::cout << "Harmonic mean of " << x << " and " << y << " is " << z << '\n'; std::cout << "Enter next set of numbers <q to quit>: "; } std::cout << "Bye!\n"; return 0; } EXCEPTIONS
11. EXCEPTIONS
12. class bad_hmean final { private: double v1; double v2; public: bad_hmean(double a = 0, double b = 0) : v1(a), v2(b){} void msg() const; }; inline void bad_hmean::msg() const { std::cout << "hmean(" << v1 << ", " << v2 << "): " << "invalid arguments: a = -b\n"; } USING OBJECTS AS EXCEPTIONS
13. USING OBJECTS AS EXCEPTIONS ... double hmean(double a, double b) { if (std::abs(a+b) < epsilon) { throw bad_hmean(a, b); } return 2.0 * a * b / (a + b); } ...
14. USING OBJECTS AS EXCEPTIONS ... int main() { double x, y, z; std::cout << "Enter two numbers: "; while (std::cin >> x >> y) { try { // start of try block z = hmean(x,y); std::cout << "Harmonic mean of " << x << " and " << y << " is " << z << '\n'; std::cout << "Enter next set of numbers <q to quit>: "; } // end of try block catch (const bad_hmean& bh) // start of exception handler { bh.msg(); std::cout << "Try again.\n"; } // end of handler } std::cout << "Bye!\n"; return 0; }
15. USING OBJECTS AS EXCEPTIONS ... double gmean(double a, double b) { if (a * b < 0) { throw bad_gmean(a, b); } return std::sqrt(a * b); } ...
16. class bad_gmean final { private: double v1; double v2; public: bad_gmean(double a = 0, double b = 0) : v1(a), v2(b){} std::string msg() const; }; inline std::string bad_gmean::msg() const { return "gmean() arguments should be >= 0\n"; } USING OBJECTS AS EXCEPTIONS
17. USING OBJECTS AS EXCEPTIONS int main() { double x, y, z; std::cout << "Enter two numbers: "; while (std::cin >> x >> y) { try { // start of try block z = hmean(x,y); z = gmean(x,y); } // end of try block catch (const bad_hmean& bh) // start of catch block { bh.msg(); std::cout << "Try again.\n"; continue; } catch (const bad_gmean& bg) { std::cout << bg.msg(); break; } // end of catch block } ... }
18. EXCEPTION SPECIFICATIONS C++98 // Deprecated since C++11. // Removed since C++17: compiler issues warning C5040. double bad_gmean(double a) throw(bad_gmean); // may throw bad_thing // exception // Since C++17: is alias for noexcept(true) double func(double a) throw(); // doesn't throw an exception
19. EXCEPTION SPECIFICATIONS C++11 // Since C++11 double func(double a) noexcept; // doesn't throw an exception double func(double a) noexcept(true); // doesn't throw an exception
20. UNWINDING THE STACK
21. КАК ЭТО РАБОТАЕТ? • При генерации исключения с помощью throw начинается раскрутка стека. • У всех объектов всех функций на стеке вызываются деструкторы. • Раскрутка останавливается, если найден подходящий обработчик исключения.
22. UNWINDING THE STACK class demo final { private: std::string word; public: demo (const std::string & str): word(str){ std::cout << "demo " << word << " created\n"; } ~demo(){ std::cout << "demo " << word << " destroyed\n"; } void show() const{ std::cout << "demo " << word << " lives!\n"; } };
23. UNWINDING THE STACK double means(double a, double b) { double am, hm, gm; demo d2("found in means()"); am = (a + b) / 2.0; // arithmetic mean try { hm = hmean(a,b); gm = gmean(a,b); } catch (const bad_hmean & bh) // start of catch block { bh.msg(); std::cout << "Caught in means()\n"; throw; // rethrows the exception } d2.show(); return (am + hm + gm) / 3.0; }
24. UNWINDING THE STACK int main() { double x, y, z; demo d1("found in main()"); while (std::cin >> x >> y) { try { z = means(x,y); } catch (const bad_hmean& bh) { bh.msg(); std::cout << "Try again.\n"; } catch (const bad_gmean& bg) { std::cout << bg.msg(); break; } } d1.show(); return 0; } Console: demo found in main() created 6 12 ???
25. UNWINDING THE STACK int main() { double x, y, z; demo d1("found in main()"); while (std::cin >> x >> y) { try { z = means(x,y); } catch (const bad_hmean& bh) { bh.msg(); std::cout << "Try again.\n"; } catch (const bad_gmean& bg) { std::cout << bg.msg(); break; } } d1.show(); return 0; } Console: demo found in main() created 6 12 demo found in means() created demo found in means() lives! demo found in means() destroyed 6 -6 ???
26. UNWINDING THE STACK int main() { double x, y, z; demo d1("found in main()"); while (std::cin >> x >> y) { try { z = means(x,y); } catch (const bad_hmean& bh) { bh.msg(); std::cout << "Try again.\n"; } catch (const bad_gmean& bg) { std::cout << bg.msg(); break; } } d1.show(); return 0; } Console: demo found in main() created 6 12 demo found in means() created demo found in means() lives! demo found in means() destroyed 6 -6 demo found in means() created hmean (6, -6) : invalid arguments: a = -b Caught in means() demo found in means() destroyed hmean (6, -6) : invalid arguments: a = -b Try again. 6 -8 ???
27. UNWINDING THE STACK int main() { double x, y, z; demo d1("found in main()"); while (std::cin >> x >> y) { try { z = means(x,y); } catch (const bad_hmean& bh) { bh.msg(); std::cout << "Try again.\n"; } catch (const bad_gmean& bg) { std::cout << bg.msg(); break; } } d1.show(); return 0; } Console: demo found in main() created 6 12 demo found in means() created demo found in means() lives! demo found in means() destroyed 6 -6 demo found in means() created hmean (6, -6) : invalid arguments: a = -b Caught in means() demo found in means() destroyed hmean (6, -6) : invalid arguments: a = -b Try again. 6 -8 demo found in means() created demo found in means() destroyed gmean() arguments should be >= 0 demo found in main() lives! demo found in main() destroyed
28. ~B() ~A() Caught 123 class A { public: ~A() { std::cout << "~A()" << std::endl; } }; class B { public: ~B() { std::cout << "~B()" << std::endl; } }; void cc() { throw 123; } void bb() { B b; cc(); } void aa() { A a; bb(); } int main() { try { aa(); } catch (int v) { std::cout << "Caught " << v << std::endl; } }
29. class problem final {...} void func(){ ... if(oh_no){ problem oops; throw oops; } ... } ... try{ func(); } catch(problem& ex){ ... } MORE EXCEPTION FEATURES Different objects.
30. #include <iostream> #include <string> #include <cctype> #include <exception> using namespace std; class INNException : public std::exception {}; class WrongLengthException : public INNException {}; class WrongCharsException : public INNException {}; class WrongChecksumException : public INNException {}; unsigned long long int parseINN(const string &s) { if (s.length() != 10) throw WrongLengthException(); if (!all_of(s.begin(), s.end(), ::isdigit)) throw WrongCharsException(); static int coeffs[] = { 2, 4, 10, 3, 5, 9, 4, 6, 8 }; int res = 0; for (int i = 0; i < 9; ++i) res += (int(s[i]) - int('0')) * coeffs[i]; if (int(s[9]) - int('0') != (res % 11) % 10) throw WrongChecksumException(); return stoull(s); } INN validation
31. int main() { string s; if (getline(cin, s)) { try { auto inn = parseINN(s); // например, 5445264092 cout << "Хороший, годный ИНН: " << inn << endl; } catch (WrongLengthException &) { cerr << "ИНН имеет неверную длину!" << endl; } catch (WrongCharsException &) { cerr << "ИНН содержит недопустимые символы!" << endl; } catch (WrongChecksumException &) { cerr << "У ИНН неверная контрольная сумма!" << endl; } catch (INNException &) { cerr << "ИНН никуда не годится!" << endl; } } } Exception Handling
32. // ... using namespace std; class INNException : public std::exception {}; class WrongLengthException : public INNException { public: const char *what() const override noexcept { return "неверная длина"; } }; class WrongCharsException : public INNException { public: const char *what() const override noexcept { return "неверные символы"; } }; class WrongChecksumException : public INNException { public: const char *what() const override noexcept { return "неверная контр. сумма"; } }; unsigned long long int parseINN(const string &s); int main() { string s; if (getline(cin, s)) { try { auto inn = parseINN(s); // например, 5445264092 cout << "Хороший, годный ИНН: " << inn << endl; } catch (INNException &exc) { cerr << "Ошибочный ИНН (" << exc.what() << ")!" << endl; } } } Other way
33. void process_file(const char *fn) { FILE *fp = fopen(fn, "r"); try { // file processing } catch (...) { fclose(fp); throw; } fclose(fp); } MORE EXCEPTION FEATURES Catch any type exception
34. class FilePtr { FILE *fp; public: FilePtr(const char *fn, const char *access) { fp = fopen(fn, access); } FilePtr(FILE *fp) { this->fp = fp; } ~FilePtr() { if (fp) fclose(fp); } operator FILE *() { return fp; } }; void process_file(const char *fn) { // Принцип RAII: получение ресурса есть инициализация // техника управления ресурсами через локальные объекты FilePtr f(fn, "r"); // …просто используем f // файл в любом случае будет закрыт автоматически } RESOURCE ACQUISITION IS INITIALIZATION RAII
35. • Деструктор вызывается только для полностью сконструированного объекта! • Если в конструкторе было исключение, то будет утечка памяти: class Y { int *p; void init() { /* здесь бабах! */ } public: Y(int s) { p = new int[s]; init(); } ~Y() { delete p; } }; EXCEPTIONS IN CONTRUCTORS Bad code!
36. class Z { std::vector<int> p; void init() { /* здесь бабах! */ } public: Z(int s): p(s) { init(); } ~Z() = default; }; EXCEPTIONS IN CONTRUCTORS Good code! if "init" throws an exception, the destructor of "p" will be called.
37. • Использовать иерархию классов исключений: не кидать int, 
 const char * и т.п. • Делать классы исключений простыми. • Использовать исключения только для исключительных ситуаций. • Генерируя исключение, понимать где и кем оно будет обработано. BEST PRACTICES
38. STD EXCEPTIONS
39. КОНЕЦ ВОСЬМОЙ ЛЕКЦИИ throw EndOfLecture();